1. Technical Field
The present invention relates to an image forming apparatus, which automatically performs an image correction in low-density areas, and to an image forming method for controlling such apparatus.
2. Description of Related Art
When forming images by an image forming apparatus, information included in an original image may be altered according to gradation characteristics of the image forming apparatus. Therefore, gamma corrections have conventionally been conducted in order to correct the inherent gamma value of the image forming apparatus, to approximate the original image.
In a usual printer, at an area where the input value just rises from 0 (zero) the output value hardly increases, in an intermediate area the output value increases exponentially, and in a area where the input value has risen to a certain extent the increase of the output is curved. This exhibits an S shaped characteristic curve. In order to approximate the gamma value of this gradation characteristic to 1, a gradation correction curve is applied, which is symmetrical to the characteristic curve of the printer with respect to a line having a gradient of 1 and passing the coordinate origin. This is the conventional gamma correction to realize a linear printer gradation (see for example, Patent Document 1).
Patent Document 1: Unexamined Japanese Patent Application Publication No. H07-230213
However, in low-density areas, where the density is lower than in the intermediate area, a color rise, in which a color is started to be printed on paper, does not occur until the input value increases to be a certain large value. And it is needed to make the correction value for correcting the low-density area to be greater than that for other areas. In cases where the aforementioned gradation correction curve is applied for all density areas, when the gradation correction curve is smoothened in order to make a proper correction for all the density areas, the low-density area receives no color, and when the gradation curve is adjusted such that the low-density area does receive color, the smoothness of the curve is degraded resulting in improper correction. Primarily for this reason, it has been difficult to automatically correct the gradation in low-density areas.
Conventionally, in order to correct the output of the low-density area, some appropriate value is manually inputted to apply a low-density correction curve onto the low density area, the corrected image is outputted, an operator visually checks whether the outputted image is within an expected output range, and when it is not within the expected range the operator again inputs another correction value on which the operator have applied an appropriate change, then a re-corrected color is outputted and checked. These operations have been repeated until the outputted image falls at least within the expected range. These operations are time consuming and somewhat complicated, and since the input value is determined sensuously by the operator, it has been difficult to achieve optimal correction in a short time.
The present invention was conceived in view of these problems and the object thereof is to provide an apparatus and a method to stably output an image with proper gradation characteristic in low-density areas by automatically performing, in addition to the usual gamma correction, a low-density correction which corrects a low-density image, an image in a density area lower than mid-density areas, the low density image being difficult to correct with a usual gamma correction. Specifically, the object is to lessen the labor of the operator and to perform quick and exact corrections by applying a low-density correction curve to automatically adjust a rising position in the gradation curve and the gray balance in the low-density areas.